Method of producing rare earth-cobalt permanent magnet using special cooling rates

ABSTRACT

A method of producing a rare earth-coabalt permanent magnet having high coercive force wherein powdery alloy containing rare earth elements and cobalt as principal component is compacted in magnetic field, the resulting green body is sintered at the temperature between 1,000* to 1,200*C, the sintered body is cooled from the above sintering temperature to quenching temperature in the range of 875* + OR - 50*C at a rate of 7*C/min. or slower rate, and then further cooled successively from the quenching temperatures to room temperature at a rate of 20*C/min. or faster rate.

Yamakawa et a1.

METHOD OF PRODUCING RARE EARTH-COBALT PERMANENT MAGNET USING SPECIALCOOLING RATES Inventors: Kazuo Yamakawa, Tokyo; Tohru Oka, Kumagaya;Masaaki Tokunaga, Kumagaya; Takeshi Mizuhara, Kumagaya; Chitoshi Hagi,Kumagaya, all of Japan Assignee: Hitachi Metals, Ltd., Tokyo, JapanFiled: July 10, 1973 Appl. No.: 377,919

Foreign Application Priority Data July 12, 1973 Japan 47-69106 US. Cl148/103, 148/3157, 148/105 Int. Cl. 1101f l/02 Field of Search 148/103,105, 108, 101,

References Cited UNITED STATES PATENTS 4/1930 Karcher 148/112 5/1972Westendorf et a1. 148/103 1 Mar. 25, 1975 3,682,714 8/1972 Martin148/3157 3,684,593 8/1972 Benz et a1. 148/103 3,755,007 8/1973 Benz etal...... 148/101 3,790,414 2/1974 Tawara et a1. 148/3157 OTHERPUBLlCATlONS Buschow, K. et al., Perm. Mag. Mtls of Rare Earth CobaltCompounds, in Zeit, fur Ang. Physik, 26, 1969, pp. l57l60.

Primary ExaminerWalter R. Satterfield Attorney, Agent, or Firm-Stewartand Kolasch, Ltd.

[57] ABSTRACT A method of producing a rare earth-cobalt permanent magnethaving high coercive force wherein powdery alloy containing rare earthelements and cobalt as principal component is compacted in magneticfield, the resulting green body is sintered at the temperature betweenl,OO0 to 1,200C, the sintered body is cooled from the above sinteringtemperature to quenching temperature in the range of 875i50C at a rateof 7C/min. or slower rate, and then further cooled successively from thequenching temperatures to room temperature at a rate of 20C/min. orfaster rate.

6 Claims,'3 Drawing Figures pmmgnmzsms FIG. I

100 960 QUENCHING TEMPERATURE ("0) FIG. 3

sub in iooambo 200 1.60

COOLING VELOCITY (C/min) FIG. 2

4 nab ah weoaovoo moai COOLING VELOCITY (C/ min) METHOD OF PRODUCINGRARE EARTH-COBALT PERMANENT MAGNET USING SPECIAL COOLING RATESBACKGROUND or THE INVENTION 1. Field of the Invention The presentinvention relates to a permanent magnet consisting of 'rare earthelements and cobalt.

2. Description of Prior Art Compared with anAlnico type permanent magnetand a ferrite type permanent magnet, a permanent magnet consisting ofrare earth elements and cobalt has very high magnetic energy product.Therefore, a rare earth-cobalt permanent magnet has recently beendeveloped in various institutions and factories-all over the world.

Rare earth elements for the rare earth-cobalt permanent magnet of thepresent invention are selected from lanthanide series having atomicnumber 57 to 71, yttrium and scandium according to the required magneticproperties of the magnet.

While, the magnetic properties of the rare earthcobalt permanent magnetdepend largely upon the preparation process thereof. At present, thefollowing method consisting of the following steps is often used.

i. A step wherein rare earth elements and cobalt are melted together sothat a final product, namely alloy having desired composition may beobtained.

In this case, the final product shows the best magnetic properties, whenthe alloy is the mixture of intermetallic compounds as Co,-,R (R: rareearth elements) and Co-R intermetallic compounds containing richer rareearth elements than Co R does.

The above mentioned alloy contains 55-70 percent by weight of Co and30-45 percent by weight of rare earth elements. The alloy can containslight amount of impurities and part of Co thereof can be replaced by Feor Cu.

ii. A step wherein resulting alloy is ground mechanically to powderwhose mesh is few mm in nonoxidative atmosphere. v

iii. A step wherein the resulting powdery alloy is compressed by a pressunder high pressure to obtain green body of adequate shape. In the abovemolding process, itis useful for the improvement in the magneticproperties of the magnet to give the green body magnetic anisotrophy byapplying external magnetic field in predetermined direction.

iv. A step wherein the green body is sintered at the temperature betweenl,000 to 1,200C.

v. An additional aging step proposed by Japanese laid patent publicationNo. 5604/1971 (See US. patent application Ser. No. 86,288 and 33,315)wherein the sintered green body is heated to about 900C and kept at thattemperature after sintering to improve further the magnetic propertiesof a magnet as a final product.

In general, it is preferable that a permanent magnet has as highcoercive force, residual induction and energy product as possible. It iswell known that these magnetic properties depend largely uponpreparation process thereof, above all on heat treatment process.

Therefore, the object of the present invention is to provide a'method ofproducing a rare earth-cobalt permanent magnet having excellent magneticproperties without using aging process.

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 showsthe influence of the quenching temperature on the intrinsic coerciveforce ,I-I of cobaltsamarium alloy permanent magnet.

FIG. 2 shows the influence of the cooling rate from the sinteringtemperatures to the quenching temperatures on the intrinsic coerciveforce ,I-I of cobaltsamarium alloy permanent magnet.

FIG. 3 shows the influence of the cooling rate from the sinteringtemperatures to room temperature on the intrinsic coercive force H ofcobalt-Samarium alloy permanent magnet.

DETAILED DESCRIPTION OF THE INVENTION EXAMPLE I Powdery alloy whosecomposition was Sm57.8- %Co was added to powdery alloy .whosecomposition was Sm66.2%Co to obtain mixed powdery alloy whosecomposition was Sm63.8%Co. The resulting mixed powdery alloy was groundby a vibration mill for 50 minutes to fine powder. The obtained finepowder was molded by compression in a mold under the pressure of 10tons/cm in magnetic field of 8 K0 The green body which was in the shapeof a rod about 5 mm long and about 10 mm in diameter was sintered l,l40Cfor 1 hour in an inert gas atmosphere. After the sintering wascompleted, the sintered body was cooled in a furnace from the sinteringtemperature to the quenching temperature of 900C at a rate of 6C/min.Then the sintered body was immediately and quickly cooled to roomtemperature by blasting it with argon gas at a rate of 200C/min. Themagnetic properties of the obtained sintered body of the presentinvention were determined. On the other hand, for comparisons sake, thesame powdery alloy as raw material was sintered under the sameconditions, and then the sintered body was subjected to aging at 900Cfor 2 hours and cooled to room temperature at a rate of 200C/min in anargon atmosphere. This is the conventional method of producing rareearth-cobalt magnets. The magnetic properties of the sintered body weredetermined. Table 1 shows the comparison between both determinations.

In Table 1, Br is residual magnetic flux density. H normal coerciveforce found on B-H demagnetizing curve (B: magnetic flux density, H:intensity of magnetic field), ,H intrinsic coercive force found on4rrM-H demagnetizing curve (417M: intensity of magnetization), and (Bl-Umaximum energy product.

As seen in Table 1, the permanent magnet according to the presentinvention is excellent, compared with the magnet produced by theconventional method.

EXAMPLE 2 The same mixed powdery alloy as in example 1 was sinteredunder the same conditions, and then the obtained sintered body wascooled from sintering temperature to various quenching temperatures at arate of 6C/min. in a cooling rate controlled furnace. The sintered bodythus cooled was immediately and quickly cooled successively from thequenching temperature to room temperature by blasting it with argon gasat the rate of about 200C/min. FIG. 1 shows the influence of thequenching temperatures on the intrinsic coercive force H of the obtainedsintered body.

As seen in FIG. 1, the optimum quenching temperature is 875 50C.

EXAMPLE 3.

Powdery alloy whose composition was Sm40%Co was added to powdery alloywhose composition was Sm-66.2%Co to obtain mixed powdery alloy whosecomposition was Sm63%Co. The resulting mixed powdery alloy was ground bya vibration mill for 4 hours to an average particle diameter of 3.3 m.The obtained fine powder was sintered by the same method and under thesame conditions as that and those of the example 1. And then thesintered body was gradually cooled from the sintering temperature toquenching temperature of 900C at a rate of 0.8C/min. in a controlledfurnace. The sintered body thus cooled was immediately and quicklycooled to room temperature. On the other hand, the same powdery alloywas sintered under the same conditions as mentioned above and cooled tothe quenching temperature of 900C at a rate of 16C/min. When thesintered body was cooled to the quenching temperature of 900C, it wasimmediately and quickly cooled to room temperature by blasting it withargon gas. Table 2 shows comparison between the magnetic properties ofthe magnet of the present invention and those of the magnets made by theconventional method.

As seen in Table 2, it is preferable to cool the sintered body graduallyfrom the sintering temperature to the quenching temperature.

EXAMPLE 4 The same mixed powdery alloy as in the example 3 was sinteredunder the same conditions, and cooled from the sintering temperature tothe quenching temperature of 900C at various cooling rates. When thesintered body was cooled to 900C, it was immediately and quickly cooledto room temperature at a rate of 200C/min. The graph in F 16. 2 showsthe influence of the cooling rates from the sintering temperature to thequenching temperature on the intrinsic coercive force ,H of the sinteredbody.

As seen in FIG. 2, the preferable cooling rate of the sintered body isslower than 7C/min.

EXAMPLE 5 Powdery alloy whose composition was Sm40%Co was added topowdery alloy whose composition was Pr8.6% Sm-67.3%Co to obtain mixedpowder alloy whose composition was Pr-16.7% Sm63%Co. The resulting mixedpowdery alloy was ground by a vibration mill for min. to an averageparticle diameter of 3.2 m. The obtained fine powder was compressed to agreen body by the same method as in example 1 and was sintered at l,l30Cfor 1 hour. The sintered body was cooled from the sintering temperatureto 900C in a furnace at a rate of 7C/min. After the sintered body wascooled to 900C, it was immediately and quickly cooled to roomtemperature at a rate of 200C/min. On the other hand, the same powderyalloy as raw material was sintered under the same conditions, and cooledin the furnace to 7009C at a rate of 7C/min. When the sintered body wascooled to 700C, it was immediately cooled to room temperature at a rateof 200C/min. Table 3 shows comparisons between the magnetic propertiesof the magnet of the present invention and that produced by theconventional method.

As seen in Table 3, the permanent magnet of the present invention hasexcellent magnetic properties, compared with the magnet produced by theconventional method.

EXAMPLE 6 Powdery alloy whose composition was Sm--66.2- %Co and powderyalloy whose composition was Sm40%Co were added to powder alloy whosecomposition was Pr-67.6%Co to obtain mixed powdery alloy whosecomposition was Pr23.3% Sm-63%Co. The resulting mixed powder alloy wasground by a vibration mill for 25 minutes. The obtained fine powder wascompressed to a green body by the same way as in the example 1. Thegreen body was sintered at l,l20C for 30 min, and gradually cooled to920C at a rate of 6C/min. When the sintered body was cooled to 920C, itwas immediately and quickly cooled to room temperature. On the otherhand, the same powdery alloy was sintered under the same conditions andimmediately cooled to room temperature at a rate of 250C/min. Table 4shows comparison between the magnetic properties of the magnet of thepresent invention and those a s of the magnet produced by theconventional method.

Table 4 Br BC 1"C (BH)max (KG) (KOe) (KOe) (MG-e) Magnet of presentinvention 8.25 7.80 12.5 17.0 Magnet Produced by the Conventional Method7.80 5.80 12.4 13.1

As seen in Table 4, the magnet of the present invention has excellentmagnetic properties, compared with the magnet produced by theconventional method.

EXAMPLE 7 Powdery alloy whose composition was Sm-40%Co was added topowdery alloy whose composition was Sm66.2%Co to obtain mixed powderyalloy whose composition was Sm-63%Co. The resulting mixed powdery alloywas ground by a vibration mill for 4 hours to an average particlediameter of 3.3 m. The obtained fine powder was compressed to a greenbody by the same way as that of the example 1. The green body wassintered at l,l40C for 1 hour, and gradually cooled to 900C at a rate of0.8C/min. When the sintered body was cooled to 900C, it was immediatelycooled to room temperature at a rate of 100C/min. by oil quenching. Onthe other hand, the same green body as mentioned above was sintered at1,140C for 1 hour and cooled to 900C at a rate of 0.8C/min. When thesintered body was cooled to 900C, it was immediately cooled to roomtemperature at a rate of 4C/min. by furnace cooling. Table 5 showscomparison between the magnetic properties of the magnet of the presentinvention and those of the magnet produced by the conventional method.

Table 5 Br BC 1C (BH)max (KG) (K00) (KOe) (MG-0e) Magnet of resentinvention 8.43 8.18 28.5 17.6

agnet Produced by the Conventional Method 8.25 5.41 15.0 16.0

As seen in Table 5, the magnet which was cooled at faster cooling rateof the present invention has excellent magnetic properties, comparedwith the magnet produced by the conventional method.

EXAMPLE 8 EXAMPLE 9 Alloy whose composition was Sm-63.8% was melted toobtain an ingot and the obtained ingot was preliminarily ground toprepare coarse powder.

The resulting coarse powdery alloy was ground by a vibration mill for 2%hours to an average particle diameter of 3.4 m. The obtained fine powderwas compressed to a green body by the same way as that of the example 1.The green body was sintered at 1,1 10C for 1 hour, and cooled to 830C ata rate of 0.8C/min. When the sintered body was cooled to 830C, it wasimmediately cooled to room temperature at a rate of 200C/min. On theother hand, the same powdery alloy was sintered under the sameconditions and subjected to aging process at 900C for 14 hours. Table 6shows comparison between the magnetic properties of the magnet of thepresent invention and those of magnet produced by the conventionalmethod.

Table 6 Br B"C lC (BHjmax (KG) (KOe) (KOe) (MGOe) Magnet of presentinvention 8.00 8.00 30.0 16.0 Magnet Produced by the Conventional Method7.80 7.70 12.0 15.2

As seen in Table 6, the magnet of the present invention has excellentmagnetic properties.

EXAMPLE 10 1,000C/min. by blasting it with liquid carbonic acid gas. Themagnetic properties of the sintered body was as follows:

Br 8.60 KG BC 8.60 KOe 1"C 34.0 KOe (BH),,,,,, 20.1 MG.Oe

EXAMPLE 11 temperature. Table 7 shows the magnetic properties of themagnet obtained by this method.

Table 7 Cooling rate lC (BH)max (C/min) (KOe) (MG-0e) The reason whyquenching was performed at the temperature in the range of 875 50C inthe present invention is that coercive force is decreased when quenchingis conducted at temperatures not included in the above mentionedtemperature range. The highest coercive force can be obtained whenquenching is performed at the temperatures of 850 to 900C.

On the other hand, the reason why the cooling from the sinteringtemperature to the quenching temperature is done at a rate of 7C/min orslower rate is that when the cooling rate is faster than the above rate,coercive force is decreased remarkably. When the cooling rate is 2C/minor slower, the best results can be obtained.

The reason why the cooling is performed at a rate of 20C/min or fasterrate in the quenching process is that if cooling is performed at aslower rate, coercive force is decreased. When cooling is performed at arate of 100C/min or faster in the quenching process, the highestcoercive force can be obtained.

As mentioned above in detail, the present invention is characterized inthat a green body having given composition is sintered, an graduallycooled from the sintering temperature to a given quenching temperature,and then quickly cooled to room temperature, without requiring afterheat treatment.

The magnet according to the present invention has excellent coerciveforce as well as excellent residual magnetic flux density and energyproduct.

What is claimed is:

1. A method of producing a permanent magnet having a residual magneticflux density of at least 8000 Gauss, a normal coercive force of at least8000 Oersted, an intrinsic coercive force of at least 27,000 Oersted anda maximum energy product of at least 17.6 X 10 Gauss Oersted comprisingthe steps of:

l. preparing a powdery alloy from about 55-70% by weight of cobalt andabout 30-45% by weight of at least one rare earth element, selected fromthe 8 group consisting of Y, La, Ce, Pr, Sm, Nd, Gd, Ho and Er,

2. compacting said powdery alloy in a magnetic field,

3. sintering said compact at a temperature between 1,000 to 1,200C.,

4. cooling said sintered body from said sintering temperature to aquenching temperature in the range of about 825 to 925C. at a rate nogreater than 7C./min., and

5. cooling said sintered body immediately from said quenchingtemperature to room temperature at a rate of at least 20C./min.

2. A method of producing a permanent magnet according to claim 1,wherein said sintered body is cooled from the sintering temperature tothe quenching temperature at a rate no greater than 2C/min and furthercooled immediately from the quenching temperature to room temperature ata rate of at least C/min.

3. A method of producing a permanent magnet according to claim 1,wherein said sintered body is cooled from the sintering temperature tothe quenching temperature at a rate no greater than 0.lC/min. andfurther cooled immediately from the quenching temperature to roomtemperature at a rate of at least 400C/min.

4. A method of producing a permanent magnet according to claim 3,wherein Samarium-cobalt powdery alloy is used as the powdery alloy.

5. A method of producing a permanent magnet according to claim 3,wherein samarium-praseodymiumcobalt powdery alloy is used as the powderyalloy.

6. A method of producing a permanent magnet according to claim 1,wherein said sintered body is cooled from the sintering temperature to aquenching temperature of between 850 to 900C at a rate no greater than0.lC/min., and the body thus cooled is further cooled immediately fromthe quenching temperature to room temperature at a rate of 400C/min. orfaster

1. PREPARING A POWDERY ALLOY FROM ABOUT 55-70% BY WEIGHT OF COBALT ANDABOUT 30-45% BY WEIGHT OF AT LEAST ONE RARE EARTH ELEMENT, SELECTED FROMTHE GROUP CONSISTING OF Y, LA, CE, PR, SM, ND, GD, HO, AND ER,
 1. AMETHOD OF PRODUCING A PERMANENT MAGNET HAVING A RESIDUAL MAGNETIC FLUXDENSITY OF AT LEAST 8000 OEGAUSS, A NORMAL COERCIVE FORCE OF AT LEAST8000 OERSTED, AN INTRINSIC COERCIVE FORCE OF AT LEAST 27,000 OERSTED ANDMAXIMUM ENERGY PRODUCT OF AT LEAST 17.6X106 GAUSS OERSTED COMPRISING THESTEPS OF:
 2. COMPACTING SAID POWDERY ALLOY IN AMAGNETIC FIELD, 2.compacting said powdery alloy in a magnetic field,
 2. A method ofproducing a permanent magnet according to claim 1, wherein said sinteredbody is cooled from the sintering temperature to the quenchingtemperature at a rate no greater than 2*C/min and further cooledimmediately from the quenching temperature to room temperature at a rateof at least 100*C/min.
 3. sintering said compact at a temperaturebetween 1,000* to 1, 200*C.,
 3. A method of producing a permanent magnetaccording to claim 1, wherein said sintered body is cooled from thesintering temperature to the quenching temperature at a rate no greaterthan 0.1*C/min. and further cooled immediately from the quenchingtemperature to room temperature at a rate of at least 400*C/min. 3.SINTERING SAID COMPACT AT A TEMPERATURE BETWEEN 1,000* TO 1,200*C., 4.COOLING SAID SINTERED BODY FROM SAID SINTERING TEMPERATURE TO AQUENCHING TEMPERATURE IN THE RANGE OF ABOUT 825* TO 925*C. AT ARATE NOGREATER THAN 7*C./MIN., AND
 4. A method of producing a permanent magnetaccording to claim 3, wherein samarium-cobalt powdery alloy is used asthe powdery alloy.
 4. cooling said sintered body from said sinteringtemperature to a quenching temperature in the range of about 825* to925*C. at a rate no greater than 7*C./min., and
 5. cooling said sinteredbody immediately from said quenching temperature to room temperature ata rate of at least 20*C./min.
 5. A method of producing a permanentmagnet according to claim 3, wherein samarium-praseodymium-cobaltpOwdery alloy is used as the powdery alloy.
 5. COOLING SAID SINTEREDBODY IMMEDIATEKY FROM SAID QUENCHIN TEMPERATURE TO ROOM TEMPERATURE ATARATE OF AT LEAST 20*C./MIN.
 6. A method of producing a permanent magnetaccording to claim 1, wherein said sintered body is cooled from thesintering temperature to a quenching temperature of between 850* to900*C at a rate no greater than 0.1*C/min., and the body thus cooled isfurther cooled immediately from the quenching temperature to roomtemperature at a rate of 400*C/min. or faster rate.